The present invention relates generally to transmitting antennas. More particularly, the present invention relates to omnidirectional slot-type transmitting antennas.
Broadcasting of radio and television (TV) entertainment programming can be realized with low-power transmitters and close-to-the ground antennas, but characteristically reaches geographically dispersed audiences by using high-power transmitters, tall antenna towers, low-loss transmission lines, and antennas that radiate with high efficiency. Each of these requirements for high performance imposes requirements on the rest of the system, as do the rules imposed by the Federal Communications Commission (FCC), which defines the minimum permissible quality of the transmitted signal as detected at specified distances from the transmitting antenna. TV transmitting antennas, in particular, should exhibit a low voltage standing wave ratio (VSWR) in order to avoid reflecting more than a minimal part of the transmitted signal back into the transmitter, which would cause ghosting and other defects objectionable to advertisers and to viewers of the received signal. TV transmitting antennas should also radiate efficiently, to maximize signal strength in proportion to the power (and cost) of the associated transmitter. TV transmitting antennas should also exhibit good structural properties, such as freedom from corrosion (for long life); small size, light weight, and self-supporting structure (for low tower bulk and wind loading); and rigidity (for constant radiative properties in strong winds).
Slot antennas in a variety of styles are capable of application to commercial use. Typical slot antennas, fed with radio frequency (RF) signals carried to the antennas by coaxial cables or waveguides, use the RF voltage differential across the slot to create a radiating electromagnetic wave. For example, a flat metal plate, incised with a slot of suitable dimensions, with the facing edges of the slot excited by RF of suitable frequency to opposite polarity using a suitable feed method, will radiate at right angles to the plane of the plate, with the polarization of the radiated signal the same as the axis of the RF voltage differential. The radiation pattern created by such a slot antenna is commonly referred to in the art as a xe2x80x9cpeanutxe2x80x9d shape, with equal lobes of high signal strength extending from and perpendicular to the front and back surfaces of the antenna, with signal strength decreasing as angle increases off the axis of radiation, and with the signals at the front and back of the slotted plate of opposite polarity (i.e., 180 degrees out of phase) and essentially equal magnitude. The dimensions of the slot (primarily) and the plate (to a lesser extent) determine the frequency range over which the slot can be excited to radiate; a simple rectangular slot has a single preferential frequency of radiation and performs poorly away from that frequency.
Bow-tie-style dipole antennas can in general exhibit desirable properties for broad band RF transmission and reception. With suitable dimensions, including overall span and the angle of the triangles comprising the bow-tie shape, such antennas can combine good electrical and mechanical performance. When sized for the ultra-high frequency (UHF) television broadcast band, the dimensions of a bow-tie antenna are practical for simple and inexpensive structures. xe2x80x9cBroad bandxe2x80x9d here refers to a single antenna able to operate well over a significant fraction of an octave. UHF television, for example, extends from 470 MHz to 806 MHz, which is most of an octave. Each UHF TV channel has an allowed bandwidth of 6 MHz, which is around 1% of an octave at the middle of the band. A television transmitting antenna that meets FCC requirements over a range of several channels is viewed by those knowledgeable in the art as xe2x80x9cbroad bandxe2x80x9d; a voice or data communications transmitting antenna, which would typically transmit a signal narrower in bandwidth than a TV broadcast, should exceed the range of frequencies of a similar TV antenna in order to be considered xe2x80x9cbroad bandxe2x80x9d for its duties. A properly dimensioned bow-tie antenna designed for receiving, rather than transmitting, UHF television, by way of contrast, should work adequately over the entire band, due to its less challenging performance requirements.
Multiple antennas can be combined to incorporate the properties of a single antenna and additional advantages as well. For example, two antennas with xe2x80x9cpeanutxe2x80x9d-shaped radiation patterns, mounted at right angles and driven with signals that are 90 degrees apart in phase, can exhibit an omnidirectional radiation pattern comparable to that of a mast dipole, the reference standard. Similarly, two horizontally polarized antennas stacked vertically at a spacing of one wavelength and driven in phase can exhibit a propagation pattern that has greater directivity than a mast dipole, which means a thinner beam of strong signal, reinforcing the tendency for the signal strength near the horizontal to be stronger than that at a downward or upward angle. Adding more antenna elements in this stack can continue to increase the directivity, which can translate to increased reception range for a given transmitter power output.
Combining the designs described above can produce a broadcast antenna comprising a vertical stack of horizontally polarized, crossed bow-tie slot antenna modules. If sized for UHF, such an antenna can be made sturdy and capable of efficiently radiating the power levels needed for commercial broadcast transmission, including television and general communications. Such an antenna can provide an omnidirectional radiation pattern to provide coverage of a commercial user""s required service area. Such an antenna can be designed to work well across a broad range of frequencies, rather than requiring unique dimensions for each frequency, potentially offering cost savings. Accordingly, it is desirable to provide a novel stacked omnidirectional crossed bow-tie slot antenna capable of handling commercial power levels, featuring ruggedness, scalable directivity, and the potential for low VSWR at low cost.
Design concepts existing in the prior art have been combined in a novel and useful way by the present invention, wherein, in a first aspect of the invention, a crossed bow tie slot antenna is comprised of a first conductive rectangular panel, a first bow-tie-shaped slot (two identical isosceles triangles with a common axis of symmetry through their unequal vertices, said vertices proximal to each other, with a parallel-sided slot joining the triangles symmetrically, the figure oriented with the referenced axis vertical) that pierces the first panel, a second conductive rectangular panel rigidly and conductively attached to the first panel at their common vertical axis of symmetry, so that the two panels cross at right angles to each other, and a second bow-tie-shaped slot that pierces the second panel.
In another aspect of the invention, an apparatus for broadcast of ultra-high frequency (UHF) television signals is comprised of first means for radiating a first RF signal with xe2x80x9cpeanutxe2x80x9d-shaped pattern of signal strength versus azimuth, with horizontal polarization, and with power levels compatible with city-wide reception from a single radiative source, as defined by the Federal Communications Commission (FCC); second means for radiating a second RF signal with xe2x80x9cpeanutxe2x80x9d-shaped pattern of signal strength versus azimuth in quadrature with the first means, collocated with the first means, adjusted in phase with respect to the first means so that the combined radiation pattern of the two means constitutes an omnidirectional transmission and meets FCC requirements for public-carrier broadcasting; means for omnidirectional radiation characterized by mutual impedance of elements that results in a voltage standing-wave ratio (VSWR) across the UHF band suitable for broadcast applications; and plurality of means for omnidirectional radiation, so configured as to provide increased power-handling capacity and increased directivity, translating to increased signal strength over a reception region.
In still another aspect of the invention, a radiation method for broadcast transmission is comprised of the steps of accepting a broadcast signal; converting the broadcast signal from a single signal to two signals in quadrature; distributing each of the two quadrature signals uniformly with regard to power and equally with regard to phase among a plurality of signal loads; carrying the load signals to radiative devices via coaxial cabling located in neutral planes; and applying the transmission signals to the radiative devices, such devices consisting of pairs of center-joined, bow-tie-slotted panel radiators, all panels lying in one of two planes in quadrature, each panel radiator in each pair driven by the outer and inner conductor of a coaxial cable with phasing corresponding to its spatial orientation compared to the rest of the radiators.
There has thus been outlined, rather broadly, the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described below and which will form the subject matter of the claims appended hereto.
In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting.
As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention.